Proposition 3  ([1]}, Theorem 6.1.1, \(X=c_0\) )
Let \(Q\in M_{c_0}\)  and \(P_r:c_0\rightarrow c_0 ~~(r\in \mathbb {N}\)  be the operator defined by \(P_r(x)=(x_0,x_1,\ldots , x_r, 0,0,\ldots )\)  for all \(x=(x_k)\in c_0\) . Then, we have
\(\chi (Q)=\lim _{r\rightarrow \infty }\left(\sup _{x\in Q}\Vert (I-P_r)(x)\Vert  \right)\) 
